1. Field of the Invention
The present invention relates to a multi-piece golf ball having a multi-layered core covered with a polyurethane cover.
2. Description of the Related Art
An ionomer resin cover is mainly used for a cover of a golf ball employing a vulcanized rubber sphere as its core because the ionomer cover has superior durability. A golf ball with the ionomer resin cover, however, gives a golfer a larger impact upon shot than do a golf ball with Balata rubber cover, and hence tends to impart an inferior shot feeling to the golfer.
In attempt to improve the shot feeling imparted by the golf ball with the ionomer cover, Japanese Patent No. 2709950, for example, has proposed a mixed ionomer cover formed from a mixture of a hard ionomer comprising a sodium salt or zinc salt of an olefin-unsaturated carboxylic acid copolymer and a soft ionomer comprising a sodium salt or zinc salt of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester terpolymer. Blending of the soft ionomer with the hard ionomer makes it possible to render the shot feeling soft, but on the other hand, sacrifices the advantages inherent in the ionomer cover, such as the repulsion property as well as the abrasion resistance (chunking property) of the golf ball.
In recent years, polyurethane has been focused on as an inexpensive cover material that imparts to the golfer a shot feeling analogous to that imparted by the Balata cover and has higher durability than the Balata cover. For example, the following golf balls having polyurethane covers have been proposed.
Japanese Patent No. 2662909 has disclosed a polyurethane cover formed by curing a urethane prepolymer with a slow-reactive polyamine curing agent, in view of a problem that it is difficult to mold the polyurethane cover because the rapid reaction between the urethane prepolymer and polyamine curing agent results in a precipitous increase in viscosity. The use of the slow-reactive polyamine curing agent suppresses a steep increase in viscosity which would otherwise occur due to the reaction rapidly proceeding between the urethane prepolymer and the conventional polyamine curing agent. However, it is still difficult to mold the polyurethane cover, because of the rapid increase in viscosity, depending on the some type of urethane prepolymer, some kinds of curing agent, some combinations of urethane prepolymer and curing agent, or the like. Even when the polyurethane cover could be formed, the repulsion property, spin performance and chunking property are not sufficient. Thus, further improvements are required.
Japanese unexamined Patent Publication No. H09-215778 has proposed a cover using a thermoplastic polyurethane elastomer. The cover using a thermoplastic polyurethane elastomer is superior in moldability to the two-part curing type polyurethane cover, but is inferior in wear resistance, tear strength and abrasion resistance (chunking property) to the two-part curing type polyurethane cover or the ionomer cover, due to the lack of three-dimensional crosslinking points.
The present invention has been achieved in view of the foregoing circumstances. It is therefore an object of the present invention to provide a multi-piece golf ball with a two-part curing type polyurethane cover, which is satisfactory in moldability and the golf ball properties such as abrasion resistance and a flight distance.
According to the present invention, there is provided a multi-piece golf ball comprising
a core including a center and at least one intermediate layer covering the center; and
a polyurethane cover covering the core, wherein the polyurethane cover has a thickness of not more than 1 mm and is formed from a cured composition which contains an isocyanate group-terminated urethane prepolymer having a residual polyisocyanate monomer content of not more than 0.1% by mass and an aromatic polyamine compound;
the core has an outer diameter of from 40.8 mm and 42.2 mm;
a difference in Shore D hardness between the central portion and the surface portion of the center is not less than 15;
a difference in Shore D hardness between the central portion of the center and the surface portion of the core is not less than 20; and a Shore D hardness at the surface portion of the core is higher than a Shore D hardness at the surface portion of the center. The foregoing and other objects, features and attendant advantages of the present invention will become apparent from the reading of the following detailed description.